1. Field of the Invention
The present invention relates to a sheet stacking apparatus, an image forming system, and a sheet stacking method and, particularly, to a sheet stacking apparatus that, upon discharge, aligns and stacks a sheet-like recording medium (in this specification, simply referred to as a “sheet”), such as a conveyed sheet, recording sheet, transfer sheet, or OHP sheet, an image forming system that includes the sheet stacking apparatus and an image forming apparatus, such as a copier, printer, facsimile machine, or digital multifunction peripheral, and a sheet stacking method performed by the sheet stacking apparatus.
2. Description of the Related Art
Conventional sheet handling apparatuses (here, referred to as the “sheet post-handling apparatuses” that perform post handling) have been well known and widely used. The sheet post-handling apparatuses perform post handling, i.e., various operations including alignment, binding, folding, or bookbinding, on sheets discharged from an image forming apparatus. In recent years, there has been a greatly increasing demand for the capability of this type of sheet post-handling apparatus to handle with sheets. Particularly, color image forming apparatuses are more likely to perform printing on sheets (hereafter, referred to as coated sheets) that are coated so as to give improved image appearance for catalogs or leaflets. However, the coated sheets generally have the following characteristics:
1) the degree of smoothness of the surface is high;
2) the adhesion between sheets is high; and
3) the Clark stiffness is low.
Therefore, there is a possibility that the stackability of coated sheets is degraded due to the above characteristics.
There is a well-known apparatus that uses a pressing member to stack discharged sheets at a proper position, whereby the degradation in stackability is prevented. With such an apparatus that uses a pressing member, it is possible to prevent the leading sheet from being pushed out by a subsequent sheet. However, in the case of thin sheets that have low stiffness, the stiffness of sheets is not enough to prevent the adhesion between the sheets, and therefore the sheet is conveyed while the sheet adheres to another sheet. As a result, the sheet is sometimes bent. For example, Japanese Patent Application Laid-open No. 2011-57313 discloses the invention in which a blower fan is provided to form an air layer in the gap between the leading sheet and the subsequent sheet so as to prevent the sheet discharged into the stack unit from adhering to the sheet already stored in the stack unit.
The invention disclosed in Japanese Patent Application Laid-open No. 2011-57313 makes consideration to prevent a degradation in the efficiency of sheet conveyance and discharge by means of the provided blower fan. However, it discloses only the installation of the blower fan. Therefore, although air blowing is effective to prevent the adhesion, there is no assurance that a sufficient air layer is formed which covers the leading end of the discharged sheet. As a result, sheet adhesion and bending occur, and alignment accuracy is degraded.
Therefore, there is a need to ensure that sheet adhesion and bending are prevented and desired alignment accuracy is obtained.